1. Field of the Invention
The present invention relates to an electro-optical modulation device which makes use of the optical double refraction characteristics of nematic liquid crystals or ferroelectric liquid crystals.
2. Related Background Art
In general, a liquid crystal display device has a pair of substrates which are placed in parallel with each other and a liquid crystal filled in the space between these substrates. The optical characteristics of these liquid crystal are selectively changed in response to, for example, the electric field applied thereto, thus performing display of information.
FIG. 1 shows, in schematic cross-section, a twisted liquid crystal display device. In this Figure, the reference numeral 1 denotes a pair of transparent insulating substrates (electrode substrates) made of a transparent material such as glass or a plastic. Transparent electrode films 2, mainly composed of tin oxide or indium oxide, are formed on the electrode substrates 1. The electrode films 2 are formed in any desired pattern. A reference numeral 3 designates orientation control films which are formed on respective electrode pattern surfaces so as to promote the orientation of the liquid crystal. The orientation control film 3 is formed by preparing a polymeric film having high insulating power and effecting an orientation treatment on the film. The reference numeral 4 designates a liquid crystal filled in the device. Polarizers 5, which are adhered to the outer surfaces of the electrode substrates 1, are arranged in the same direction as the direction of orientation treatment of the orientation control films 3. In the liquid crystal display devices shown in FIG. 1, the substrates are arranged such that the directions of orientation of the orientation control films 3 orthogonally intersect each other when viewed in plan.
When no voltage is applied between the electrode plates, the molecules of the liquid crystal filled in the liquid crystal display device are twisted by 90.degree., thus exhibiting optical rotatory power. However, as electric voltage is applied to the liquid crystal display device, the molecules of the liquid crystal are rearranged substantially through 90.degree., so that the optical rotatory power is extinguished. Since the orientation films provided on the upper and lower substrates are arranged such that the directions of orientation thereof is orthogonal mutually. Therefore, any incident light is interrupted when the voltage is applied, so that an image with contrast between bright and dark is formed on the display surface.
The liquid crystal display device of the type mentioned above, however, suffers from a disadvantage that, since the display is not visible unless the voltage is applied, the content and the pattern of the display cannot be recognized by the user when the voltage is not applied to the device.
In order to obviate these problems, several methods have been proposed such as provision of color filters on the outer surfaces of the substrates of the liquid crystal cell, as well as color printing of the image of the pattern to be displayed. These proposals, however, are still unsatisfactory. For instance, the method employing the filter encounters a problem in that, due to a certain thickness of the substrate, the filter boundary is not readily recognized when the display device is observed obliquely.
It has been proposed also to form a colored transmitting film on the transparent electrode film which in turn is formed on the substrate. This method also suffers from disadvantages such as unevenness in the color, fluctuation in the density, change in color due to heat, and fading of color due to exposure to light.